The present invention relates to a process for the extrusion of films of vinylidene fluoride polymers, to a process for the manufacture of piezoelectric films from films obtained by the abovementioned process, and also to equipment for carrying out the abovementioned process.
It is known at the present time to produce films of vinylidene fluoride polymers which possess noteworthy piezoelectric properties and which are particularly suitable for the electronics industry.
Thus, British Pat. No. 1,339,295 (KUREHA KAGAKU KOGYO K.K.) describes a technique leading to polyvinylidene fluoride films possessing good piezoelectric properties, which consists in subjecting a film of this polymer, consisting preponderantly of crystals of the beta type, to a direct polarising voltage of between 200 and 1,500 KV/cm. According to the said document, it is not possible to produce the starting film directly by extrusion and cooling, because this technique always leads to films essentially containing crystals of the alpha type, which are therefore unsuitable. To obtain a suitable starting film, it is necessary, according to the said document, to subject a cooled extruded film to an additional monoaxial or biaxial stretching operation at a temperature below 130.degree. C., and preferably below 50.degree. C., so as to convert the crystals of the alpha type to crystals of the beta type. This stretching treatment must be carried out carefully and at a low speed in order to ensure the desired crystallographic conversion. In order to simplify the manufacturing process, it generally suffices to subject the film of alpha crystallinity to longitudinal stretching. However, it seems that this monoaxial stretching carried out at low temperature leads to a deterioration in the transverse mechanical properties of the film, and the latter may even tend to fibrillate. Moreover, the film has an inadequate tear strength after nicking. The films thus obtained therefore have unsatisfactory mechanical properties.